Fiber-optic current sensors have been described in References 1 to 7 identified in the list of references below. The references may hereinafter be identified in bracketed numbers (e.g., [1]) corresponding to the order in which the references are identified in the list of references. Generally, such a sensor consists of a sensor head, an optoelectronic measuring unit and a fiber cable in between. The sensor head includes a sensing fiber wound around the conductor whose current I is to be measured, and an optical retarder connected to the sensing fiber. The optical retarder causes a differential phase shift of approximately 90°, thereby converting between the linearly polarized light in the connecting fiber and the circularly polarized light in the sensing fiber. The magnetic field of the current I introduces a phase shift between the left and right circularly polarized light waves propagating in the sensing fiber. The optoelectronic measuring unit contains a light source and a light detector as well as a signal processor. Light from the light source is sent through the connecting fiber to the sensor head. Light returning from the sensor head is processed and measured by the light detector, and a signal S is derived therefrom. The signal processor calculates the current I from the signal S.
Generally, this calculation requires the knowledge of calibration data of the current sensor in order to account for the response and non-linearities of the sensor.
This type of sensor is often used in high-voltage environments, such as a substation of an electricity transmission and distribution network, and the sensor head is mounted at a high-voltage potential. For example, it may be placed on top of a free-standing electric insulator column or integrated in other high-voltage equipment, such as a circuit breaker. The optoelectronic measuring unit is placed at ground potential, for example, in an outdoor cabinet near the circuit breaker or in the substation control room.